1. Field of the Invention
The present invention relates to an apparatus for processing a picture signal, and particularly to an apparatus for controlling a quantization level to be modified by a motion vector of pictures.
2. Brief Discussion of the Related Art
Recently, in order to enhance picture quality, there have been used well-known circuit in order to code a picture signal into digital data. However, in the case where the picture signal is coded into digital data form, a large volume of data should be coded. Accordingly, efficient compression of data is required.
Data compression is performed using various redundancy factors in the picture data so that data is included in a given band, in various systems such as HD-VCR, digital VCR, digital camcorder, HD-TV and video phone, etc. An intra frame coding and an inter frame coding methods for eliminating the redundancy data are known.
FIG. 1 is a block diagram illustrating a conventional coding apparatus for coding a picture signal into digital data. Referring to FIG. 1, a pre-filter 10 allows a low frequency signal to be passed, and reduces a high frequency signal to take advantage of the fact that man's visual characteristic is more sensitive to a low frequency than a high frequency. A motion vector detector 12 calculates the difference between a frame-delayed picture signal in a frame memory 32 and a picture signal input from the pre-filter 10 and outputs it. That is, the motion vector detector 12 calculates motions between a picture block within a present frame and a corresponding picture block within a previous frame as horizontal and vertical components MVH and MVV, and outputs them. At this time, the motion vector detector 12 detects the motions between the present frame and the previous frame in a macroblock unit shown in FIG. 2. In FIG. 2, the macroblock is composed of 16.times.16 or 32.times.16 pixels. A motion compensation unit 14 compensates the picture information of the previous frame stored in the frame memory 32 according to the motion vector components MVH and MVV, which are detected from the motion vector detector 12. A first adder 16 receives the picture signal of the present frame which is output from the pre-filter 10 and has the high frequency component than that of the previous frame which is motion-compensated in the motion compensation unit 14, and generates a difference value between two signals. A Discrete Cosine Transform DCT 18 performs a DCT in a block unit of 8.times.8 pixels for the signal generated from the first adder 16. When the picture signal of a spatial domain is transformed into a frequency domain by the DCT, the picture signals, in general, are distributed close to the low frequency domain. A quantizer 20 quantizes the picture signal transformed in the DCT 18 according to a predetermined quantization level. The quantized picture signal is zigzag-scanned by a run length coder (not shown), and is represented by a pair of sequential numbers of `0` level and level not `0. For one example, when a level of a quantized first signal is `10`, the run length coded signal is represented as "(0,10)". Also, if a signal level of `5` follows two signal levels of `0` after a signal level of `10`, the run length coded signal is expressed by "(2,5)".
Run length coded signal includes an End of Bit. A variable-length coder 22 compresses the quantized and run length coded picture signal. That is, a large number data among signal levels (0.about.255 levels) represented by 8 bits is denoted by a number of small bits, while a small number of data is represented by a number of large bits, thereby the total bits to denote the picture signal is decreased. Generally, in the case where run length coding is performed after quantizing the discrete cosine transformed picture signal, there are a large number of signal levels representing `0` and a small number of large signal levels. Accordingly, `0` is represented by small bits, while 255 is denoted by large bits, and therefore the total number of bits is reduced. Since the data length which is compressed and output in the variable-length coder 22 is not constant, a buffer 24 stores temporary data and outputs it at a constant rate. Then, the buffer 24 determines the quantization level with respect to the fullness and outputs a quantization level signal to the quantizer 20. That is, in the case where the fullness of the buffer is high, a high quantization level makes the data amount for coding decrease. While the fullness is low, a low quantization level allows the data for coding to be increased. The quantization level is, in a slice unit, determined as shown in FIG. 2.
An inverse quantizer 26 reproduces the picture signal quantized in the quantizer 20 to an original signal prior to quantizing. An inverse DCT (IDCT) 28 reproduces the output signal of the inverse quantizer 26 to reproduce the picture signal prior to performing the discrete cosine transform DCT. A second adder 30 adds the output signal of the inverse DCT 28 and that of the motion compensation unit 14. Therefore, the output of the second adder 30, which is similar to a picture signal before adding it to the output signal of the motion compensation unit 14 by the first adder 16, is input to a frame memory 32. When a picture signal of the following frame is input to the pre-filter 10, the input signal becomes a pictures signal of the present frame. On the other hand, the picture signal to the frame memory 32 becomes that of the previous frame. The above operation is repeatedly performed.
In the system described above, since the quantization level of the quantizer 20 is controlled solely by the fullness of the buffer, there have been limitations in enhancing the efficiency of data compression.